Not since the first Designer’s Forum articlededicated to geocomposite design (Richard-son and Zhao, GFR June/July 1998) hasthere been any further discussion in themagazine regarding the collection of land-fill gas below covers. There were, in fact,significant errors in the portion of that ar-ticle that dealt with landfill gas pressure dis-sipation. Since that time there have beenseveral other literature references that havepresented more complete and correct de-sign approaches for relieving landfill gaspressures below landfill covers (e.g., Thiel1998; Richardson and Zhao 1999; Bachus etal. 2004). This article updates GFR’s seriesregarding designing with geocomposites—see Part 1 (January/February 2005) for com-plete GFR bibliography of geocomposite-related articles since 1998—and expandsthe documented design applications of geo-composites in landfills by describing howthey can be utilized for side slope seep col-lection in modern bioreactor-style landfills.

Landfill gas pressure dissipationLandfill gas (LFG) is continuously gener-ated in a landfill as the waste decomposes.Ideally, landfill gas would be withdrawnfrom the landfill at the rate it is generated.

level, such that the computed factor ofsafety against slope stability failure is ac-ceptable, and gas collection out of the top

of the landfill is facilitated; thus, reducingthe potential for downward pressure gra-dients of VOCs.

Design methodology forgas control beneathfinal coversThe method for designing gas venting lay-ers below landfill final covers was originallydeveloped by Thiel (1998). The primarydesign criterion for geocomposites is to pro-vide enough flow capacity to reduce thelandfill gas pressure to an acceptable level interms of factor of safety for slope stability.The fundamental design equations for a typ-ical cover slope configuration (Figure 1),repeated from the references, are as follows:

Equation 1

Designer’s Forum

Update on designing with geocomposite drainagelayers in landfills—Part 2 of 4: geocomposites onbioreactor landfill sideslopes to control seeps and gas

By Richard Thiel and Dhani Narejo

The concern for final landfill cover designsincorporating geomembranes is that an up-lift pressure can be caused by the gas. Froma slope-stability point ofview, gas pressure is anexcess pore pressure thatserves to reduce the ef-fective normal stress. Thispressure results in a de-crease in the effectivestress beneath the finalcover geomembrane that,ultimately, can lead toslope stability failure orsurface “whales” (Photo1). Furthermore, inade-quate gas venting imme-diately below new coversystems on old landfillshas been identified as thecause of sudden increasesin groundwater monitor-ing volatile organic com-pound (VOC) inputs, be-cause the LFG is beingconstrained to migratedown and laterally outward. The primarypurpose of the geocomposite in a landfillgas collection system is to provide flow ca-pacity to maintain the landfill gas pressurewithin the geocomposite at an acceptable

GFR

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γcover•hcover•cosß -(FSs•γcover•hcover•sinß)

tanδ

umax =

Photo 1. Geomembrane gas bubble during final cover construction.

Figure 1. Infinite slope stability equation withgas forces.

20

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where umax = allowable gas pressure (kPa);γcover = cover soil density (kN/m3), hcover= soil cover thickness (m); FSs = factor ofsafety against sliding; δ = interface frictionangle (degrees) for geocomposite-geomem-brane interface; β = slope angle.

The value of the calculated maximumallowable gas pressure then controls the de-sign of the gas relief system that can be de-signed based on the geometry shown in Fig-ure 2. The design equation for the gaspressure that might exist within a drainagegeocomposite and pipe network can be cal-culated as follows (Thiel 1998):

Equation 2

where qg = landfill gas supply flux estimatedby the designer or in accordance with Thiel(1998) (m3/m2/sec); D = slope distancebetween strip drains (m); and θgreq =required transmissivity of gas drainage layer(m3/sec per m width); γg = unit weight ofgas (kN/m3).

For lined landfills that do not recirculateleachate, the gas generation rate, rg, can beconservatively assumed to equal 0.1

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Incoming Gas Flux = φg

Strip Drain(typ.)

Unit Width

A A

D

a) Plan

Geomembrane

Strip Drain(typ.)

Transmissivity = ψg

Gas Flow InGas – Relief Layer

Towards Strip Drain

X

X =

0

b) Section A – A

8(g-max)U =

DL = D / 2

Figure 2. Model of gas flow to strip drains.

18” Vegetative Cover

18” Soil Barrier layer(k < 10-5 cm/s)

Perforated PE UnderdrainPipe to Collect Seepage Underlain

by Geomembrane Rub Sheet

Waste and InterimCover Soil

Geocomposite forGas Venting andSeep Collection

Geocomposite Drain

Geomembrane

Perforated LateralPE Pipe at 150 O.C. to Vent

Gas and Drain Seeps

Figure 3. Seep collection at toe of gas collection layer under final cover system.

umax =qg•γg

θgreq

D2

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Equation 5

where θreq = required hydraulic (i.e., water) transmissivity for geonet or geocomposites (m3/sec per m width); ugas = dynamic vis-cosity of landfill gas (kPa); uwater = dynamic viscosity of water (kPa); γwater = unit weight of water (kN/m3); and γgas = unit weight of gas (kN/m3).

Design example

Assume an average waste height of 60 ft. with an average density of 50 lb./ft.3 (We will use English units here because of their prevalence in this situation and the gas generation rate units.) Also, assume that slope stability calculations indicate that the maximum allowable gas pressure for the cover systems is 6 in. of water column (1490 N/m2) for a gas venting pipe spacing of about 30 ft. (10 m). Assume the density of landfill gas is 12.8 N/m3. Calculate the required hydraulic transmissivity of the venting layer.

SolutionCalculate the surface gas flux as:

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qg = 0.1 x 50 x 60 = 300 ft.3/ft.2/yr. = 2.9E(-06) m3/m2/s

Calculate required transmissivity from Equation 4:

Note that this gives the required gas transmissivity. From Equation 5 we know that the equivalent hydraulic transmis-sivity is approximately 10 times the gas transmissivity. Therefore the equiva-lent required water transmissivity is ap-proximately 3.1 x 10-6 m2/s. Applying reduction factors for creep, chemical and biological clogging, and an overall factor of safety would likely result in a design specification of approximately 2 x 10-5 m2/s for a geocomposite long-term in-soil transmissivity.

Seep collection

A complementary function served by the gas venting layer is the collection of side slope seeps. This can be especially relevant for landfills in high-precipita-

θreq = θgreq ugas γwater

uwater γgas=10•θgreq ˜

θgas = (2.9 x 10-6)(12.8)

1490102

8= 3.1 x 10-7 m2/s

Photo 2. Geocomposite placed directly on waste interim cover—below final cover construction.

scf/year/lb. of MSW. A conservative gas flux at the surface could be calculated as:

Equation 3qg = rg γwaste Hwaste

where rg = the gas generation rate; γwaste = unit weight of the waste; and Hwaste = average height of the waste.

Equation 2 can be re-arranged to cal-culate the required transmissivity of gas drainage layer relative to the pipe spacing as follows:

Equation 4

Notice that the above equation provides required transmissivity for the flow of gas—not water. Transmissivity tests in the labo-ratory, however, are performed using water as the test fluid. To compare the measured performance of drainage layers, the required transmissivity from Equation 4 must be con-verted to an equivalent water transmissivity, or vice versa. This is accomplished with the help of the relationship between transmis-sivity, viscosity and density as shown:

θgreq =qg•γg

umax

D2

8[ ]

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tion areas or bioreactor landfills where liquid is added to the waste or where daily cover layers are not periodically removed or breached to promote vertical percolation of liquids. The seeps would be collected at the toe of the geocompos-ite gas collection layer (Figure 3).

Uncontrolled lateral seepage can manifest itself by distressed vegeta-tion, unsightly wet zones on the landfill sideslopes, local slope instabilities, the release of leachate and an increase in odor. Although it may be possible to explicitly design a drainage layer just for these seeps, the authors have found that adequate seep collection capacity is pro-vided, even in situations where seeps are a chronic problem, if a drainage compos-ite is designed to facilitate gas collection in accordance with the procedures de-scribed above. A case history illustrates the significance of this issue.

Case historyThe authors were involved with a project that involved partial closure on one face of a landfill that had received leachate recirculation. The landfill had been ex-posed to approximately 40 in. per year of rainfall; in addition, a significant por-tion of the waste had received leachate recirculation on an experimental basis at a rate of between 50–75 gallons per ton of waste.

As one of the landfill subcells reached final grades sideslope seeps began to develop. Waste filling operations were moved to another location and approxi-mately 5 acres of 3(H):1(V) slope face were to be final-closed with barrier, drainage and soil layers very similar to the schematic cross section illustrated in Figure 3. A photo of the geocompos-ite placed directly on the waste interim cover is shown in Photo 2. The underd-rain pipes, installed approximately every 150 ft. directly below the geocomposite for the collection of gas and seeps, are shown in Photo 3. In the period of the first year after the closure was completed, the gas underdrain pipes yielded approximately 30–50 cubic feet per minute of gas, and over 2 million gallons of seepage liquids were collected (a steady 4 to 5 gpm); these continue to be collected. In addition, the

Photo 3. Vent and leachate collection seep pipes installed belowgeocomposite.

underdrain system allowed the cover to be installed with none of the typical problems related to gas-billowing of the geomem-brane during construction.

This case history illustrates the benefit of installing an underdrain layer below landfill final covers, especially where land-fill gas and side slope seeps may exist. Ob-servations suggest that if the underdrain layer transmissivity is designed for the col-lection of landfill gas in accordance with the suggested method, then the transmis-sivity will be adequate for the landfill seeps as well.

Designer's Forum

ReferencesBachus, R., Narejo, D., Thiel, R., and Soong,

T-Y. 2004. GSE Drainage Design Manual. GSE Lining Technology Inc., Houston.

Koerner, R.M., Soong, T.Y. 1998. “Analy-sis and design of veneer cover soils.” Proceedings of the Sixth International Conference on Geosynthetics, Atlanta. IFAI, Roseville, Minn., pp. 1–26.

Richardson, G. and Zhao, A. 1998. “Com-posite drains for side slopes in land-fill final covers.” GFR, vol. 16, no. 5,pp. 22–25.

Richardson, G. and Zhao, A. 1999. Design of Lateral Drainage Systems for Landfills. Tenax Corp., Baltimore, Md.

Thiel, R.S.1998. “Design Methodology for a Gas Pressure Relief Layer Below a Geomembrane Landfill Cover to Improve Slope Stability.” Geosynthetics Interna-tional, vol. 5, no. 6, pp. 589–617.

Richard Thiel is president of Thiel Engineer-ing, Oregon House, Calif., www.rthiel.com.

Dhani Narejo is a drainage product manager for GSE Lining Technology Inc., Houston, www.gseworld.com.

Copyright © 2005 GFR magazine. Reprint with permission of IFAI.

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